In certain applications, it is desired to monitor the oxygen content of gases more or less continuously. A system which does this effectively may be used, for example, to monitor the gaseous effluent from industrial boilers, kilns, ovens, internal combustion engines, driers, heat treating furnaces, incinerators, refinery process units, gas turbines scrubbers and the like. Based on the information so gleaned, the amount of oxygen present may be adjusted to desired levels. For example, the air being introduced into the combustion phase of a boiler may be regulated to achieve optimum efficiency, and to reduce nitrous oxide and/or sulphur dioxide emissions. Insufficient oxygen, which can cause too high smoke opacity, slag buildup, boiler tube fouling, decreased heat transfer, excessive maintenance, and wasted fuel, may be avoided, as also can an excess of oxygen which can cause waste of energy by heat loss. Such monitoring systems can also be adapted to act as sources to enable feedback systems to effectuate such adjustments automatically and continuously.
One means for so monitoring the oxygen content of such gases works on the principle of a zirconium oxide fuel cell oxygen sensor. Typically, such sensors consist of a ceramic tube made from zirconium oxide that has been stabilized with yytrium and has porous platinum electrodes coated opposite each other on both its inner and its outer surfaces at its sensing end. When the cell is heated to a temperature above 600.degree. C. (1100.degree. F.) the ceramic material becomes permeable to oxygen ions. Vacancies in its lattice structure permit such ions to pass through it, thus rendering the cell into an oxygen ion conducting solid electrolyte. When the number of oxygen molecules per unit volume is greater at one plate of the cell than at the other, oxygen ions will migrate from the former to the latter. The platinum electrodes on each side of the cell wall provide catalytic surfaces for the change of oxygen molecules into oxygen ions and vice versa. Thus, oxygen molecules entering the cell through an electrode gain electrons to become ions which enter the electrolyte. Simultaneously, at the other electrode, the oxygen ions lose electrons and are released from the surface of the electrode as oxygen molecules. This flow creates an electron imbalance which produces a voltage potential between the electrodes. The magnitude of that potential is a function of the temperature of the cell and the relative partial pressure on each side of the cell. Partial pressure is defined as the pressure exerted by each component in the mixture that goes to make up the gas. It may be calculated as follows: EQU Pi=Ni.times.R.times.T/V
where
pi=the partial pressure, PA1 Ni=the number of molecules of the specie, PA1 R=the Universal Gas constant, PA1 T=temperature (absolute scale), PA1 and V=volume. PA1 T=Absolute temperature of cell PA1 R=Universal Gas Constant PA1 F=Faraday's Constant PA1 P1=Partial Pressure of oxygen in the reference gas PA1 P2=Partial Pressure of oxygen in the monitored gas PA1 C=a Cell Constant for each individual cell, PA1 and Ln(P1/P2) is the natural logarithm of ratio P1/P2.
The partial pressure of the component gas "i" is then the same as if it occupies the same volume at the same temperature in the absence of other gases.
The relationship between the oxygen partial pressure at the monitored side and that at the reference gas side (typically air, which is 20.95% oxygen by volume), the temperature, voltage output, and cell constant of the individual cell are defined by the "Nernst" equation as follows: EQU E=RT/4F.times.Ln(P1/P2)+C
where E=Voltage out
The electrical voltage output of such a cell may be utilized, for example, in a closed loop combustion control system which uses the oxygen sensor output signal to "trim" the fuel/air mixture ratio. In such systems, the readings for actual stack gas oxygen content are compared with a desired set point. An appropriate control system output is generated to adjust automatically the fuel/air ratio, by such means as changing the amount of combustion air to the burner and/or the amount of fuel admitted to the burner, adding diluents and/or oxygen, etc., thereby assuring that the desired oxygen set point is maintained. The desired effect of this is to achieve optimum combustion efficiency by regulating exactly the amount of oxygen to achieve complete combustion of the fuel. The Nernst equation "C" factor is to adjust output readings for peculiarities of the individual cell, such as those which may be induced by physical characteristics, damage, and/or conditions which are unique to that particular cell. Its value may be determined and utilized to correct the output readings by such means as a solenoid actuated bypass conduit through which reference gas may be periodically introduced to the environs of the outer electrode surface. The "reading" while that is being done will then be that of the "C" factor alone; all other factors affecting the cell output at that instant thereby having then been eliminated or at least so minimized as to have no material effect on the resulting readings.
It is also desired to effectuate such monitoring by comparable means and mechanisms to those previously described in situations where ambient phenomena may induce false output readings which render the cell ineffectual as a practical matter. One example of this is where there is a differential between the absolute pressure of the gas at one of the electrodes from that at the other. An even more difficult example is where such a differential in absolute pressure is pulsating. Thus, in an internal combustion diesel engine, the effluent gas not only is at an absolute pressure differential with respect to the reference, but also, because of the valving in the engine, that pressure pulsates through the exhaust system. Conditions of this type can cause a sensor of the type described to exhibit erroneous readings.
An explanation of this begins by noting that the principle involved in the operation of cells of the type described above is the migration of oxygen ions through the walls of the cell that is motivated by a disparity in the oxygen pressure as between the two masses of gas. To be an accurate indicator of the proportion of oxygen content in the test gas, this migration, and the resulting imbalance in electrical potential, should be solely the result of the differential between the partial pressure of the oxygen in the gas that is on the test side and that of the oxygen in the gas that is on the reference side of the cell. However, the magnitude of such migration may be influenced as well by a differential in the absolute pressure of the reference environment and that of the environment being sampled. The reason for this result is that the effect of an increase in absolute pressure is to compress the gas and thus "densify" it; i.e., to concentrate a greater number of oxygen molecules into an equivalent volume. But the sensor is capable only of reacting to oxygen pressure differentials. Its outer electrode now being exposed to more oxygen molecules over the same area for that electrode due to the "densification" which has occurred, the sensor "reads" this as if the composition of the test gas had changed through an increase in the proportion of its oxygen constituent, which it has not. To interpret the resulting output as indicative of an oxygen partial pressure differential reading therefore, is erroneous since it has not been factored to take this absolute pressure differential element into account. While theoretically it might be possible to factor the cell output to take such absolute pressure differentials into account, this would be an unrealistically complex approach as a practical matter. In a pulsating pressurized environment particularly, such as that present in an internal combustion engine exhaust system, not only is such an aberration produced by the addition in absolute pressure, but the aberration produced is unstable since the added pressure so applied is constantly changing due to the exhaust valve operation of the engine. It will be apparent then why, although the use of devices of the type described is desired, that has not been practical in some situations because of the inaccuracy of the results produced.
Accordingly, it is an object of this invention to provide means to detect the oxygen content of a gaseous environment.
Another object of this invention is to provide such means for use in monitoring such oxygen content on a more or less continuous basis.
Yet another object of this invention is to provide means for achieving one or more of the foregoing objectives that is adapted for use in contexts wherein such gaseous environments are at differing absolute pressures.
Still another object of this invention is to provide means for achieving one or more of the foregoing objectives that is adapted for use in contexts wherein such gaseous environments are pulsating.
Yet another object of this invention is to provide means for achieving one or more of the foregoing objectives that is adapted for producing repetitive output signals to be utilized as corrective feedback sources.
Another object of this invention is to provide means for achieving one or more of the foregoing objectives that is adapted for use in internal combustion engines.